1. Field of the Invention
The present invention relates to a seismic isolating bearing for supporting a structural body for protection against earthquakes, and more particularly to a seismic isolating bearing for isolating a wide variety of vibrational energies ranging from minor vibrations induced by road traffic to medium- and large-scale earthquake vibrations.
2. Description of the Related Art
Various seismic isolating bearings for use between structural bodies, such as between buildings and the ground have been proposed to protect the structural bodies against earthquakes and other vibrations.
One typical seismic isolating bearing comprises a laminated body for supporting vertical loads, the laminated body comprising viscoelastic layers and stiffener plates that are alternately arranged in a vertical direction.
The laminated body has a first coupling on its lower end for coupling the laminated body to a member fixed to a foundation on the ground, and a second coupling, on its upper end, for coupling the laminated body to a structural body that is to be supported.
The laminated body also has a vertical central hole in which there is fitted a cylindrical lead block which is complementary in shape to the central hole. The lead block serves as an energy absorber.
Operation of the conventional seismic isolating bearing will be described below. If the lead block were dispensed with, the laminated body would function as follows:
When the first coupling would move horizontally with respect to the second coupling due to an earthquake, the laminated body would allow such a horizontal movement to a considerable extent through its own elastic deformation (shearing deformation).
In building applications, such a seismic isolating bearing, with no lead block, would be effective and useful to some extent. However, the laminated body would be substantially deformed under the pressure of strong winds applied to the building. With respect to a more serious problem, since the elastic energy of the laminated body can be stored, it would possibly permit greater vibrations to be applied to the building, which the laminated body supports, when an earthquake occurs than to buildings that are fixed directly to ground, resulting in larger damage to the building.
The above problems are solved when the laminated body is combined with the lead block.
The lead block bears the wind-induced load applied to the building for protecting the laminated body against shearing elastic deformation. When a relatively strong earthquake occurs, the lead block is plastically deformed itself, dissipating a portion of the applied earthquake energy as thermal energy. In this sense, the lead block serves as an energy absorber.
The function of the laminated body as an elastic support and the function of the lead block as an energy absorber are combined with each other for high isolating performance.
However, it is known that the physical properties of the lead block vary upon repeated elastic deformation, resulting in deterioration of its function as an energy absorber.
To determine configurations and dimensions of a lead block, therefore, it is necessary to design the lead block so that it will perform the best function as an energy absorber under most critical conditions.
More specifically, the lead block is required, from a design standpoint, that it be elastically deformed only when it is hit by a relatively strong earthquake, i.e., the lead block should be thick and sturdy.
Seismic isolating bearings capable of functioning effectively, without fail, against large vibrations induced by strong earthquakes and also smaller vibrations with relatively weak vibrational accelerations caused by relatively weak earthquakes, heavy traffic conditions, and the like, should be highly useful.
However, since the highly rigid lead block cannot be deformed by weak vibrational accelerations, the conventional seismic isolating bearing is not effective to absorb vibrations induced by relatively weak earthquakes.
Another problem is that the equivalent stiffness of the conventional seismic isolating bearing does not match medium vibrations caused by medium-scale earthquakes.
The amount of plastic deformation that the lead block undergoes when subjected to medium vibrations is smaller than that which is induced by large vibrations caused by stronger earthquakes. Therefore, the equivalent stiffness of the seismic isolating bearing against medium vibrations is larger than against large vibrations.
Inasmuch as the equivalent stiffness of the seismic isolating bearing is established to match strong earthquakes, it is too high for medium vibrations. Therefore, the natural period of vibration of the system, which is composed of the building and the seismic isolating bearing, is too short to isolate the building from medium-scale earthquakes.